1. Field of the Invention
The present invention relates to a method for treating and compressing surfaces having areas difficult to access, more specifically the axial retaining hooks of turbomachine blades including a groove between the hook and the foot of the blade.
2. Description of the Related Art
In a gas turbine aeronautical engine, the axial retaining hooks of the blades in the housing of said blades on a turbine disk and the rims of the turbine disks including a radial groove axially retaining the blades are strongly stressed. The blade hooks undergo a high level of static stresses, with regard to the grooves of the disks, so there are contact and wear problems between the disk and the flange applied against the face of the disk.
Currently, to improve their mechanical performance, these parts are surface treated, by conventional shot-blasting, in order to enhance their fatigue and corrosion resistance.
The prestressing shot-blasting operation is a mechanical treatment intended to improve the properties of a metal part by surface hardening. It is based on the structural transformation of the materials. The conventional method consists in placing the mechanical parts under surface compression, by the projection of small steel, glass or ceramic balls. This shot-peening operation creates a compressed area which is the seat of internal compression stresses through which the resistance is increased.
According to an example of conventional shot-blasting, the surface is hammered by projecting steel balls BA 315 (steel balls with a diameter of 0.315 mm) with an intensity F15A (according to the Almen index). A gaseous flux is used, produced by expansion through a nozzle, then the nozzle is moved, parallel to the surface of the part, or the part is moved relative to the nozzle, to cover the surface to be treated.
Given the difficulty in accessing certain areas, this type of shot-blasting cannot be done in optimal conditions. As it happens, the shot-blasting jet cannot be directed directly onto the surface and the shot-blasting is done by bounce, in the best cases.
Bounce shot-blasting is much less effective because the balls arrive at the surface with a weaker kinetic energy. Also, in some cases, the compression level is not sufficient to treat the surface of the part.
Furthermore, conventional shot-blasting does not give an assurance of a good coverage of the areas difficult to access such as the blade grooves or even the disk grooves.
Nor is the use of the laser shock compression method applicable to these areas. As it happens, since these areas are concealed, they cannot be accessed by the laser beam.
Laser shock treatment is a method that aims to generate plasticizing shock waves in a material, in order to also improve its surface properties. The shock waves are obtained by focusing on the surface of the material a very intense laser impulse (GW/cm2) in the presence of a containment medium over very short periods (a few nanoseconds). The treatment is likely to induce residual compression stresses to thicknesses reaching several hundreds of micrometers, and do so on a wide variety of materials, in particular for the applications that are of interest in the field of steels, aluminum alloys or titanium. The treatment is used to improve the surface properties, such as fatigue, wear or even corrosion resistance. One of the benefits of this technique lies in the fact that the surface conditions of the parts are almost unchanged.
The applicant was set the objective of treating surfaces on a hook axially retaining a turbomachine blade having areas of gas turbine engine parts difficult to access by using the ultrasound shot-blasting method.
The ultrasound shot-blasting method makes it possible to compress and thus harden the surface layers of metal materials, the aim of this technique being to improve the life of the parts. The method consists in causing a sonotrode to vibrate, at frequencies close to the ultrasound frequency, via acoustic elements, linked to a generator. Balls of different kinds are propelled toward the material that has to be shot-blasted, via the sonotrode.
In order to overcome the drawbacks of the conventional surface treatment methods on areas difficult to access, the invention consists in applying the ultrasound shot-blasting method to said areas, of blade groove type, for which the methods such as conventional shot-blasting or laser shock do not allow complete coverage of the surface.